High-speed synthesized field focus coil

ABSTRACT

A high speed magnetic focus lens develops a relatively large static magnetic force field which has radial symmetry and which varies along the axis of an electron beam and radially outwardly therefrom so as to focus off-axis electrons to the same focal point on a cathode ray tube screen, thereby producing a visible spot characterized by minimum distortion and aberration and therefor which is very small. The lens also develops a smaller dynamic field component which varies with beam deflection so as to maintain the beam in focus with minimum aberration, even though the beam is deflected through relatively large angles while moving over the screen. The static and dynamic elements of the lens are electrically isolated so as to minimize the time constant associated with dynamic focusing, thereby enabling the lens to focus a beam moving at high speed.

United States Patent Gabor 15] 3,686,527 [451 Aug. 22, 1972 [54] HIGH-SPEED SYNTHESIZED FIELD 3,225,269 12/1965 Worcester ..3l5/5.35 FOCUS COIL 3,237,059 2/1966 Meyerer ..3l3/84 X 3,345,529 10/1967 Loefi'ler et al. ..3l3/84 [72] Amherst 3,411,033 11/1968 King ..31s/31 [73] Assignee: Sanders Associates, Inc., Nashua,

NH. Primary Examiner- Malcolm F. Hubler 22 Filed: Dec. 12,1969 At'omey-wms Ellmge [21] Appl. No.: 884,714 [57] ABSTRACT Related U,S. Application Data {a high speed magnetite focufs 11315 cklleviellgps a gaistively arge static magnetic orce 1e w ic as ra i sym- [6,3] gg ig gg g g t Apnl metry and which varies along the axis of an electron a an one beam and radially outwardly therefrom so as to focus off-axis electrons to the same focal point on a cathode [52] US. Cl. ..315/31 R, 313/84, 312455.355, ray tube screen thereby producing a visible spot [51] Int. Cl 29/66 characterized by minimum distortion and aberration [58] Field 6 5 and therefor which is very small. The lens also develops a smaller dynamic field component which varies with beam deflection so as to maintain the beam in focus with minimum aberration, even though the [56] References C'ted beam is deflected through relatively large angles while UNITED STATES PATENTS moving over the screen. The static and dynamic elements of the lens are electrically 1solated so as to 2,472,165 6/1949 Mankm ..3l5/3l TV i i the i constant associated with dynamic 2 focusing, thereby enabling the lens to focus a beam mo h d 2,995,680 8/1961 Moulton ..315/31 TV mg a g Spee 3,323,000 5/1967 Mancebo ..315/ 14 X 21 Claims, 9 Drawinglfig res CENTER FIELD as 16 43. 136 I h 64 H4 x DEFLECTION ABERRATLONIOO 02 SYSTEM 98 'FOC s ADJ U 84 86 B8 1" 33M 1, 94 E 1 92 I I 1 I l l Patented Aug. 22, 1972 5 Sheets-Sheet 1 F I G. I

CONTROL SYSTEM DEFLECT FOCUS CONTROL SYSTEM WIIIIII II I 1 I l r I Lg] WILLIAM D GABOR AT 'ERNEY BY 3 I I F I G 2A 5 Sheets-Sheet 2 INVENTOR WILLIAM D. GABOR ATT RNEY on mwm mom m mwm 3 8w mvm mm 2% Patented Aug. 22, 1972 Patented Aug. 22, 1972 3,686,527

5 Sheets-Sheet 4.

I500 :52 DEFLECTION X W SYSTEM QM lsog 'vw INVENTOR F|G.6 WILLIAM 0. GABOR ATTORNEY Patented Aug. 22, 1972 3,686,527

5 Sheets-Sheet 6 FIG.7 I

INVENTOR w ABQR ATTORNEY HIGH-SPEED SYNTHESIZED FIELD FOCUS COIL BACKGROUND OF THE INVENTION This application is a continuationin-part of my copending application Ser. No. 724,893, filed Apr. 29, 1968, now abandoned.

I Field of the Invention This invention relates to an improved magnetic lens system for focusing an electron beam. It relates more particularly to a magnetic focus lens for use in a high performance cathode ray tube display system.

Large screen projection cathode ray tube displays are being used more and more as random plot displays. The capability and flexibility of these systems depend to a great extent on the speed at which they can trace a given symbol. It is highly desirable to achieve writing speeds on the order of 100,000 inches/second or more. In order for a system to display high quality symbols of the required size, e.g. a 3 inch by 3 inch image, at these high speeds, it is essential that the trace width on the tube screen (i.e. the diameter of the electron beam where it impinges the screen) have a minimum width, e.g. on the order of 3-4 mils.

Moreover, the beam current in the cathode ray tube must be kept at a relatively low peak value, e.g. on the order of 3-4 milliamperes. In order to meet both of these requirements, the display system must employ high speed, high performance magnetic focusing and deflection systems, the former being my major concern in this invention.

2. Description of the Prior Art There are two types of conventional magnetic focus lenses. The first is simply a single small winding which encircles the neck of the cathode ray tube. A fixed electrical current is passed through the winding, thereby producing a magnetic field in the tube neck which runs generally parallel to the axis of the tube. The second type of lens comprises a solenoid having a uniformly distributed winding. The solenoid also encircles the tube neck and carries a fixed current. When energized, it also produces a static axial magnetic field in the tube.

Neither of these types of conventional lens is satisfactory for use in displays of the type with which we are concerned here, because they cannot focus with sufficiently low focusing aberration to obtain the desired trace width. Present day focus arrangements are disadvantaged also because the electron beam becomes defocused when it is deflected through relatively large angles. This defocusing, which results from the increased path length when the beam moves away from the tube axis, is particularly troublesome when the tube has a short depth of focus. Consequently, the display may appear blurred, somewhat distorted and of nonuniform intensity.

SUMMARY OF THE INVENTION Therefore, it is an object of this invention to provide an improved magnetic focus lens system for use in a high speed, high perfomiance random plot display system.

A further object of the invention is to provide an improved magnetic focus lens which produces a minimum amount of focusing aberration.

Another object of the invention is to provide a magnetic focus lens system which dynamically focuses an electron beam so that it remains in focus even though it is deflected through relatively large angles.

Still another object of the invention is to provide a dynamic magnetic focus lens which operates at a high speed and has a short settling time.

Still another object of the invention is to provide an improved magnetic focus lens which requires a minimum amount of power.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

Briefly, my lens system comprises a generally cylindrical electromagnetic lens structure which encircles the neck of a cathode ray tube. The lens generates within the neck of the tube a static magnetic force field which has axial symmetry and which varies radially and also along the tube axis so as to focus substantially all of the electrons in the electron beam to the same focal point on the tube screen. This forms an image on the screen characterized by minimum distortion and aberration.

In addition, the lens system develops a smaller dynamic force field component within the neck of the tube which varies as the beam is deflected from the longitudinal axis of the tube. This dynamic focusing correction assures that the electron beam will remain in focus so that the trace width remains very small even for relatively large symbols.

In one preferred embodiment of my invention, the magnetic lens takes the form of a set of separate windings spaced closely apart along a common axis which coincides with the tube axis. All but one of the windings are connected to a source of direct current. These windings have an ampere turn distribution which yields the shaped magnetic force field mentioned above.

The remaining winding in the lens provides the dynamic focus correction. It receives a signal from a lens control system which varies with beam deflection and has the effect of varying the overall'focal length of the lens so that the lens keeps the beam in focus with minimum aberration over a wide range of deflection angles.

In another lens embodiment, the large, static magnetic field is produced by a permanent magnet structure encircling the neck of the cathode ray tube. The structure is magnetized so that it develops an axially varying magnetic force field similar to the one resulting from the lens employing a set of static field Both lens embodiments have an outer sleeve constructed of a low magnetic permeability material such as ferrite to confine the magnetic field within the tube. This not only increases the efficiency of the lens, but also it minimizes the interaction between the lens on the one hand and the deflection yoke and the cathode ray tube gun on the other, which might tend to distort the focus and deflection fields.

My lens is able to focus an electron beam having a relatively large diameter, on the order of 0.3 inch so that it forms on the tube screen a visible image which is substantially free of aberration and distortion and therefore has a very small diameter, e.g. 3-4 mils. Furthermore, the lens keeps the beam in focus even though the beam sweeps over a relatively large area, e.g. 3 inches square, so that the entire display has a uniform trace width on the order of 3 4 mils.

In both lens embodiments, the dynamic magnetic field component is only a small percentage of the overall field produced by the lens. Also, the resonance frequency of the dynamic focus windings is high and there is minimum interaction between that winding and the lower resonance frequency static field components. Consequently, the dynamic focus winding has a fast time constant and focusing is accomplished at a relatively high speed.

DESCRIPTION OF THE DRAWINGS For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 illustrates a cathode ray tube display employing a magnetic focus lens system made in accordance with this invention;

FIG. 2 is a perspective view with parts broken away showing in more detail elements of my improved lens;

FIG. 2A is a fragmentary sectional view of modified form of ferrite sleeve used in the FIG. 2 lens;

FIG. 3 is a schematic diagram of the FIG. 2 lens and its associated control system;

FIG. 4 is a graph illustrating a typical magnetic field distribution and ampere turns distribution for a lens system made according to my invention;

FIG. 5 is a vertical section of another embodiment of my improved magnetic focus lens;

FIG. 6 is a schematic diagram of the FIG. 5 lens together with its associated control system;

FIG. 7 is a side view with parts broken away of still another lens embodiment; and

FIG. 8 is a sectional view along line 8-8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, a display of the type with which we are concerned here employs a cathode ray tube 10 having a screen 10a. The gun 12 of tube 10 directs an electron beam B along the tube axis A so that the beam impinges on screen 10a. A magnetic focus lens 14 together with its control system 16 focuses the beam on a screen 10a so that it forms a small round spot at point P thereon.

A deflection yoke 18 is positioned on the neck of tube 10 just forward of lens 14. Also, a deflection control system 20 supplies vertical (Y) and horizontal (X) deflection signals to yoke 18 to deflect electron beam B so that the beam visibly traces selected symbols on screen 100. Focus lens 14 generates a strong, static magnetic field component confined within tube 10 which is shaped so that substantially all of the electrons in electron beam B strike screen 10a at the same point, e.g. point P. This produces a very small, intense round spot image at point P which is substantially free of distortion and focus aberration.

Lens 14 also develops a dynamic magnetic field component which varies in accordance with deflection signals applied by deflection control system 20 to focus control system 16. This dynamic component keeps beam B in sharp focus as it moves away from the center of screen 10a. Consequently, the trace width on screen 10a remains very small even though the beam is tracing relatively large symbols.

Referring now to FIG. 2, in one preferred embodiment of my invention, lens 14 comprises a wire-carrying bobbin indicated generally at 22. Bobbin 22 has a generally cylindrical sleeve 24 shaped to engage snugly around the neck of tube 10. A plurality of, herein 10, radially outwardly extending flanges 26 are evenly distributed along the length of sleeve 24, forming nine separate compartments 28 on bobbin 22.

Each compartment 28 contains a separate winding having a selected number of ampere turns as will be described in more detail later. There is a central or innermost winding 30 in the fifth compartment from either end; the winding 30 is flanked by windings 32a and 32b, respectively; the next outer pair are windings 34a and 34b, respectively; beyond these are windings 36a and 26b and finally, in the two end compartments 28 are windings 38a and 38b, respectively. The end compartments 28 may also carry small aberration trim windings 67a and 67b. The opposite ends of each winding form leads 44 which are connected to the focus control system 16 (FIG. 1

Still referring to FIG. 2, lens 14 also includes a generally cylindrical ferrite sleeve 50 encircling bobbin 22, which confines the magnetic field within the lens. Conveniently, sleeve 50 consists of two identical sections 50a and 50b in the shape of half cylinders which engage along mating surfaces 52. Since surfaces 52 extend generally parallel to the magnetic flux developed in lens 14, the seams there do not materially affect the field in the lens. A set of nine slots 54 are fonned in the upper mating surface 52 of section 50a to serve as electrical feedthroughs for the winding leads 44.

The ends of sleeve 50 terminate in a pair of radially inwardly extending beveled flanges 56 and 58, which overhang the opposite ends of bobbin 22. The flanges 56 and 58 have a minimum diameter only slightly larger than the diameter of the neck of tube 10.

Referring now to FIG. 3 of the drawings, focus control system 16 comprises a static field control section indicated generally at 64 which energizes windings 32 38 and a dynamic magnetic field control section indicated generally at 66 which drives winding 30. These two sections supply the total energy for the magnetic field required to focus the electron beam. The currents applied by system 16 to the various windings 30 38 in lens 14 and the number of turns in each winding are selected to yield the proper ampere turn distribution along the axis of lens 14.

FIG. 4 shows the ampere turn contributions of the windings in a typical nine winding lens. The ampere turns for each winding are indicated by horizontal steps identified by the winding numbers.

windings 32-38, driven by the static control section 64, have fixed ampere turns. The ampere turns of the central winding 30, on the other hand, varies over the range indicated by the shaded area 68 in FIG. 4, depending upon the signal applied to it from dynamic focus control section 66 (FIG. 3). The ampere turns distribution from one end of the lens to the other, i.e.

from the left-hand bar 38a to the right-hand bar 38b in FIG. 4, is selected to produce the desired magnetic flux density distribution which has radial symmetry and which has the desired variation along the axis of the lens. A typical flux density distribution for the nine winding lens is indicated by curve 70 in FIG. 4. Superimposed on curve 70 is a second curve 72 which represents the flux density distribution in a conventional, uniformly-wound solenoid-type focus lens.

A comparison of curves 70 and 72 shows that the flux density in the present lens is more uniform near the center of the lens than is the case with a conventional lens. Moreover, near the ends of the lens, the density drops off more rapidly than it does in the usual solenoid-type lens. This distribution of the flux density gives rise to a shaped magnetic force field within lens 14 whose lines of force F (FIG. 1) have more nearly parabolic curvature. This applied particularly near the ends of the lens.

Of course, the number of static field windings in the lens may vary depending upon the particular application. A five winding lens has been tested with good results. Generally, however, a larger number of windings is preferred because a smoother field distribution is produced in the lens.

The shaped force field within lens 14 brings substantially all of the electrons in beam B to the same focal point P on screen a (FIG. 1), even though beam B may have a relatively large cross section so that many of its electrons are spaced appreciably from the lens axis. Consequently, the spot image produced at point P on screen 100 (FIG. 1) is quite small and intense. Moreover, it suffers from relatively little aberration and distortion. In a sense, then, the operation of my lens may be likened to that of a parabolic reflector which can focus a relatively large beam of light to a single focal point and thus avoid spherical aberration.

Turning to FIG. 2, ferrite sleeve 50 provides a low reluctance path for the return fields developed by the lens windings. This increases the efficiency of the lens because substantially all of the magnetic flux developed by the lens is actually used to focus the electron beam. Also, since the field is confined within the lens, there is less chance of the field interacting with the fields developed by yoke 18 or with the other elements of the system. Sleeve 50 has a high permeability so that it exhibits low loss at high frequencies.

The sleeve flanges 56 and 58 at the opposite ends of the lens also affect the shape of the end fields at those points. Specifically, the value of the angles between the radial end planes of the bobbin 22 and the bevel surfaces 56a and 58a of the flanges 56 and 58 (FIG. 1) have a direct effect on the aberration-reducing qualities of the lens. In practice the angles 0 are selected to produce minimum aberration with the particular ampere turns distribution used in the lens. For a nine winding lens having the distribution shown in FIG. 4, an angle 0 of about 45 produces satisfactory results. However, for other selected ampere turns distributions, angle 0 may be much larger. In fact, it may be as large as 135, so that the ends of sleeve 50 are outwardly beveled. One advantage of a relatively small angle 0 is that the flanges then help to confine the end fields so that they are less apt to interact with other components of the system.

Actually, in some cases, sleeve 50 may becontoured along its entire length to help shape the magnetic field within the lens itself. FIG. 2A illustrates a modified sleeve 50 encircling a wire-carrying bobbin 22 like the one shown in detail in FIG. 2. The inner wall 74 of sleeve 50 is hollowed out so that it has a barrel-like contour. That is, it is spaced relatively far apart from the winding at the middle of the lens and gradually draws closer to the windings towards the end of the lens. Contouring the sleeve in this fashion produces a varying flux density along the axis of the lens in much the same way as does the distribution of the ampere turns described above. Depending on the application, one or the other or both of these techniques together may be used to properly shape the magnetic force field within the lens.

Referring again to FIGS. 3 and 4, control section 66 applies a signal to dynamic focus coil 30 which continuously alters the focal length of lens 14, as the beam B is deflected from the tube axis A (FIG. 1). Consequently, the ampere turns contribution of winding 30 may vary from a relatively low quiescent value (corresponding to the bottom of area 68 in FIG. 4) when the beam is undeflected to a higher value approximating that of the adjacent windings 32a and 3212 when the electron beam is deflected a maximum distance from the tube axis. In any event, only a relatively small change in current in winding 30 is needed to maintain the electron beam in sharp focus at all angles of deflection, even though tube 10 may have a relatively short depth of focus.

Moreover, variation of the lens center field in this fashion has relatively little effect on the spot image as far as aberration and distortion are concerned. Consequently, the beam width remains uniform and narrow as the beam moves across screen 10a, with the result that the image on screen 10a is distinct over the entire screen.

It is important to appreciate that lens 14 is able to dynamically focus the electron beam at a very high speed. That is, winding 30 changes the focal length of the lens well within the settling time of the associated deflection yoke 18. There are several reasons for this. First, dynamic focus winding 30 has relatively few turns. The means it has a relatively small inductance and a correspondingly high resonant frequency. In addition, the turns in the static field windings 32-38 are a maximum at the ends of the lens and a minimum at the center thereof near winding 30. Thus, the windings 32a and 32b which are most closely coupled to winding 30 also have a relatively low inductance and therefore do not unduly affect the inductance of winding 30. On the other hand, the windings progressively farther away from winding 30 are coupled closely to that winding and their higher inductances are therefore reflected insignificantly in the effective inductance of winding 30.

Finally, additional effective electrical isolation is obtained by dynamically damping the static field windings 32-38 and by proper design of control sections 64 and 66, as will be described later.

In actual tests, winding 30 was driven over its entire focus range, indicated by the hatched area 68 in FIG. 4, in only 500 nanoseconds. This area represents the focus change required when beam B (FIG. 1) is deflected from the tube axis A to the edge of screen 10a. This is well within the deflection and operating time of a good high speed deflection yoke 18.

Referring to FIG. 3, the static focus control section 64 is basically a regulated current source for the static field windings 32-38 and the trim windings 67a and 67b in lens 14. Section 64 comprises a dc. source illustrated by a battery 80 connected between ground and one end of a focus adjustment potentiometer 82, the other end of which is grounded. The center tap of potentiometer 82 is connected via a summing resistor 84 to the input terminal of a conventional fixed current d.c. regulator 86. A choke 88 is connected between the output terminal of regulator 86 and the outer end of the outermost winding 38a. The adjacent ends of the adjacent static field windings 32-38 are connected together and a damping resistor 90 is connected in parallel with each winding 32-38. The outer end of the winding 38b is connected to ground by way of a resistor 92 and also via a summing resistor 94 to the input terminal of regulator 86.

Resistor 92 functions as a current sampling resistor for steady state or quiescent focus current regulation. The quiescent focal length of the lens can be adjusted by means of potentiometer 82.

Similar adjustable current regulation is afforded the aberration trim windings 67a and 67b. More particularly, battery 80 is connected to one end of an aberra tion trim potentiometer 96, the other end of which is grounded. The center tap of potentiometer 96 is connected by way of a summing resistor 98 to the input terminal of a d.c. regulator 100. A choke 102, trim windings 67a and 67b and a current sampling resistor 104 are all connected in series between the output terminal of regulator 100 and ground. Also, a damping resistor 106 is connected across each trim winding 67a and 67b. To complete the loop, a summing resistor 108 is connected between the trim winding 67b at resistor 106 and the input terminal of regulator 100. Proper adjustment of potentiometer 96 regulates the current in the two trim windings for minimum aberration.

For dynamic operation, the two do. regulators 86 and 100 present high d.c. impedances. Also, the chokes 88 and 102 ensure that the circuit 64 provides a high impedance at high frequencies. Finally, resistances 90 and 106 damp the static field and trim windings. All of these factors help to isolate the dynamic winding 30 and thereby maximize the speed of dynamic focusing.

The current signal applied to winding 30 varies in accordance with the X and Y deflection signals from deflection system 20. Both deflection signals are applied via summing resistors 112a and 112k to the input terminal of each of a pair of operational amplifiers 114 and 116. A nonlinear feedback network 118 is connected between the output terminal of amplifier 114 and separate summing resistors 1120 and 112d leading back to the input terminal of amplifier 114. A similar network 120 is connected to amplifier 116 in the same fashion. Networks 118 and 120 provide a high degree of negative feedback. They also shape the output signal from amplifiers 1 14 and 116 so that winding 30 receives the correct focus signal as the beam is deflected.

More particularly, the desired focus control signal for winding 30 does not vary linearly with beam deflection. Rather, it should vary approximately as the square root of the distance of the beam from the tube axis. Also, the focus control signal should be positive no matter which way the beam is deflected from the axis.

Therefore, when the beam sweeps from one side of the screen to the other, the focus control signal should have a parabolic waveform whose minimum point corresponds to zero beam deflection. There are various well-known feedback shaping networks for accomplishing this. For example, network 118 may comprise a conventional ladder arrangement composed of diode limiters which are biased to conduct at different input signal levels in response to positive input signals. Thus, as the amplitudes of the positive deflection signals from system 20 increase, the diodes conduct in succession. Consequently, the signal at the output terminal of amplifier 114 or 116 has a waveform composed of contiguous straight line segments having gradually decreasing, absolute slopes. As more and more diode limiters are used in the feedback network, the line segments become shorter and shorter and the waveform can be shaped in this way to approach a parabolic curve. Network is similar to network 118 except that it accommodates negative deflection signals.

The output signals from amplifiers 114 and 116 are applied via a pair of summing resistors 126a and 12612 to the input terminal of an amplifier 128. Also, a center field adjustment potentiometer is connected between battery 80 and ground. The center tap of potentiometer 130 is connected by a summing resistor 1260 to amplifier 128. The output terminal of amplifier 128 leads to one end of winding 30, the other end of which is connected to ground through a resistor 132. The voltage across resistor 132, corresponding to the winding 30 current, is fed back via a summing resistor 126d to the input of amplifier 128. This also tends to make the dynamic focusing current independent of the static focusing system and thereby enhances high speed operation of the dynamic focus section.

Adjustment of potentiometer 130 regulates the quiescent level of the magnetic field component due to winding 30. That is, it determines the location of the bottom of the hatched area 68 in FIG. 4. Then, as the current in winding 30 varies depending upon the signals from deflection system 20, the dynamic focus field varies over the range indicated by area 68 in FIG. 4.

Referring now to FIG. 5, a modified lens employs a permanent magnet structure to produce the. required shaped field. More particularly, lens 140 has a generally cylindrical ferrite magnet 142 which is magnetized in such a way as to produce the same flux density variation along the lens axis described above in connection with FIGS. 1-4. More particularly, different rings or annular zones of magnet 140 are magnetized as indicated. The ditferent zones are spaced apart axially as needed to form the equivalent of the ampere turn density variation in the. FIG. 2 lens. The same effect can be produced by a series of appropriately spaced and polarized ring magnets. Thus, the magnet 142 supplies the entire static force field required to focus the electron beam with minimum aberration. Dynamic focusing in lens 140 is accomplished by a small helical winding 144 on a generally cylindrical bobbin 146 which fits snugly coaxially within magnet 142. The current applied to winding 144 varies the focal length of lens 140 much the same way as described above in connection with FIG. 2 lens.

In a preferred embodiment of lens 140, the winding 144 is bifilar. One winding filament carries a direct current which provides a steady do. or quiescent field to establish focus center; the other winding filament is driven by the dynamic focus control system. Also, separate aberration trim windings 160a and 160b may be included in lens 140 at the opposite ends of bobbin 146 as described above.

Finally, ferrite sleeve 147, which is much the same as sleeve 50 (FIG. 2), encircles magnet 142.

Turning now to FIG. 6, the control system for driving lens 140 is somewhat like the dynamic focus control section 66 in FIG. 3. The deflection signals from deflection system 20 are applied via summing resistors 150a and l50b to the input terminal of an amplifier 152. Also, a reference voltage source in the form of a battery 154 is connected to one end of a potentiometer 156, the other end of which is grounded. The center tap of potentiometer 156 is connected by a summing resistor 1500 at the input of amplifier 152. The dynamic focus winding 144 and a current regulating resistor 158 are connected in series between the output terminal of amplifier 152 and ground. The voltage across resistor 158 is fed back by way of a summing resistor 150d to the input terminal of amplifier 152. The adjustment of potentiometer 156 regulates the current in coil 144 to establish the quiescent focal length of lens 140.

A potentiometer 162 connected between battery 154 and ground regulates the current in the trim windings 160a and 160b. That is, the center tap of potentiometer 162 is connected via a summing resistor 164 to the input terminal of a dc. current regulator 166. A choke 168, trim windings 160a and 160b and a current regulating resistor 170 are connected in series between the output terminal of regulator 166 and ground. Also, a summing resistor 164 is connected between winding 16012 at resistor 170 and the input terminal of regulator 166. As described above, regulator 166 and choke 168 ensure that the circuit has a high impedance over a high frequency range to minimize electromagnetic coupling between the dynamic and static elements of the lens.

The permanent magnet lens shown in FIGS. and 6 is advantaged over the electromagnetic lens version because the major portion of the static focusing field energy is supplied by a pennanent magnet. Therefore, this lens requires less external power and needs no regulated current source to control the static field. Rather, only a small steady current to establish focus center need be supplied. Since this contributes to only a small percentage of the total field required by the lens, its regulation as far as focus stability is concerned, is less critical. An additional advantage of the permanent magnet lens embodiment is that the windings within the lens are all very high frequency windings having resonances well above the operational region of the dynamic focusing. The ferrite magnet 142 has a very low incremental permeability and therefore does not materially affect the effective inductance of focus winding 144.

Other obvious modifications may be made to my improved lens without departing from the spirit of the invention. For example, instead of using wire to form the winding turns 30-38 in the FIG. 2 lens, a relatively wide conductive ribbon may be coiled in each compartment 28. The number of turns in each coil and the width of the ribbon in each coil may then be selected to yield the above-described shaped field. This construction has the advantage of avoiding the winding problems associated I with forming each winding -38 so that it is perfectly uniform about its axis. Also the thickness of the ribbon in each compartment may be selected to yield the desired coil crosssection in each compartment 28.

The aforesaid lens construction is also advantaged because the damping resistors 90 and 106 can be incorporated in the lens itself in the form of semiconductive tape between adjacent ribbon coils. This means that the lens will have fewer external connections to control system 16 (FIG. 1

FIGS. 7 and 8 illustrate such a lens having nine coiled conductive ribbons 180, 182a, 182b, 184a, 184b, 186a, 186b, 188a, and 188b.These ribbons are coiled on a bobbin 190 similar to bobbin 22 in FIGS. 2. That is, each ribbon is wound relatively tightly about the bobbin within a compartment thereof so that each turn of the winding overlies the previous turn, i.e., like a roll of tape. Also, a ferrite sleeve 50 (FIG. 2) may encircle the ribbons.

The thicknesses, widths and lengths of the various ribbon coils are selected to yield an ampere turn distribution which is essentially the same as that of the FIG. 2 lens. Thus, the middle ribbon 180 constituting the dynamic winding is relatively short (i.e. only a few turns). The end ribbons 188a and 188b, on the other hand, are relatively wide and long to produce a stronger field at the ends of the lens for the reasons described above.

Field windings comprising these coiled ribbons are easier to from accurately then the wire windings shown in FIG. 2. This is because with the latter, each turn in the winding must be carefully laid so that it is in perfect alignment with the previous turn. Otherwise, gaps are formed in the winding which distort the field. With the ribbon windings, on the other hand, there are fewer turns involved and each turn extends the full width of the winding, so that there are no such gaps.

The FIG. 7 lens has an additional advantage in that the static focus windings comprising ribbons 182 to 188 are connected together within the lens itself. Consequently, it has fewer electrical leads extending to control section 16 (FIG. 1). More specifically, the dynamic winding ribbon 180 has two external leads 194. However, the static field windings comprising ribbons 182-188 have only two external leads 196a and 196b connecting the ends of ribbons 188a and 188b to choke 88 and resistor 92, respectively (FIG. 3). This is because ribbons 182-188 are connected together in series within a sleeve similar to sleeve 50 (FIG. 2) by leads 198.

Damping resistors (FIG. .3) are built into the FIG. 7 lens by coiling semiconductive tape 202 along with each ribbon 182 to 188. A strip of insulating tape 203 is coiled along with ribbon 180. This produces the desired shunt resistance across each winding as shown schematically in FIG. 3. Thus, only eight electrical leads extend without the sleeve; two for the dynamic winding, two for all the static windings, and four for the trim windings (not shown). This is in sharp contrast to the 22 external leads required in the FIG. 2 lens. Moreover, this is all accomplished without any material increased in the overall size or weight of the lens itself.

Damping resistors 90 (FIG. 3) may also be built into the FIG. 7 lens by shaping semiconductive tape 202 in the form of a disc and utilizing such discs as compartment separators of bobbin 190. Depending on the construction of bobbin 190, the discs could be donut shaped or solid in crosssection. The discs could be adhered to bobbin 190 as shown in FIG. 7, in which case such discs would be horseshoe shaped for ease of placement thereon. These arrangements would likewise reduce the number of external leads required as stated above.

Finally, it should be mentioned that when I use the term lens I means a magnetic structure having the capacity of bringing electrons in a beam to a focus.

It will be seen from the foregoing that my improved magnetic focus lens system greatly improves the quality of cathode ray tube displays. Thc static field component of the lens brings the electrons in the beam to a sharp focus at a single point on the tube screen. Further, the lens develops a dynamic field component which maintains the beam in sharp focus as the beam moves across the tube screen. This assures that the display of information is accomplished with a minimum trace width so that each symbol in the display is easily visible and of uniform intensity. Finally, the lens has a very fast time constant so that it can be used with display systems having very fast writing speeds.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A magnetic focus lens system for focusing an electron beam generated in a cathode ray tube comprising A. a plurality of separate windings spaced closely along an axis and arranged to engage around the neck of said tube,

B. means for supplying a dc. current to said windings, the ampere turns of said windings being distributed along said axis so as to bring substantially all off-axis electrons to a single focal point on said screen,

C. an additional winding wound coaxially with said windings,

D. means for producing deflection signals for deflecting the electron beam, and

E. means responsive to said deflection signals for applying a current signal to said additional winding which varies in accordance with the deflection of said beam.

2. A magnetic focus lens system as defined in claim 1 and further including a bobbin A. for engaging over the neck of said tube, and

B. having separate compartments for containing said windings.

3. A magnetic lens system as defined in claim 2 wherein A. said additional winding is situated in a compartment near the middle of said bobbin, and

B. said windings progressively further away from said additional winding have progressively increasing numbers of winding turns.

4. A lens system as defined in claim 1 and further including a ferrite sleeve encircling said windings so as to confine the magnetic field within the tube.

5. A lens system as defined in claim 4 wherein the opposite ends of said sleeve are shaped to give the end field magnetic force lines generally parabolic curvature.

6. A lens system as defined in claim 1 and further including A. a pair of small trim windings situated adjacent the outer ones of said windings, and

B. an adjustable current source connected to said trim windings for shaping the magnetic force lines of the ends of said lens to give them generally parabolic curvature.

7. A lens system as defined in claim 1 wherein said signal applying means comprises A. means for dynamically damping said windings,

and

B. control means connected to receive vertical and horizontal deflection signals, said control means including a signal shaping circuit which shapes the signal applied to said additional winding so that it has a generally parabolic waveform as the electron beam is deflected off the tube axis.

8. A lens system as defined in claim 1 wherein said windings comprise a plurality of separate current-carrying coiled ribbons arranged along the axis of the beam.

9. A lens as defined in claim 8 and further including a sleeve of low magnetic permeability material encircling said ribbons for confining the focus field within the lens.

10. A lens as defined in claim 8 and further including a length of semi-conductive tape rolled up with each said ribbon so as to function as a shunt resistance therefor.

l l. A lens as defined in claim 8 and further including discs of semi-conductive material mounted between and separating each of said ribbons and coupled to each of said ribbons so as to function as a shunt resistance therefor.

12. A focus system for focusing an electron beam comprising A. a magnetic focus lens for producing a static magnetic field along the axis of the beam to focus the beam on a surface,

B. means for producing deflection signals for deflecting the electron beam, and

C. means responsive to said deflection signals for varying the focal length of the lens so as to maintain the beam in focus as it moves across said surface, said focal length varying means comprising 2. a winding encircling the axis of the beam, and

2. means for applying a current signal to said winding which varies approximately as the square root of the distance which the beam is deflected from the beam axis.

13. A focus lens for focusing an electron beam on a surface comprising A. a plurality of magnetic field-producing elements spaced along the axis of the beam so as to generate a magnetic force field whose density varies along said axis and radially outwardly therefrom in a manner to focus off-axis electrons in the beam at a single focal point on said surface,

B. a sleeve of low magnetic permeability material encircling said field-producing elements for confining the focus fields within the lens, said sleeve having its opposite ends 1. overhanding said elements, and 2. beveled at an angle of approximately 45-l35 measured from the radius of said sleeve,

C. means for producing deflection signals for deflecting the electron beam, and

D. means responsive to said deflection signals for varying the focal length of the lens so as to maintain the beam in focus as it moves across said surface.

14. A lens as defined in claim 13 wherein said sleeve has a barrel-shaped inside contour.

15. A magnetic focus lens system for focusing an electron beam generated in a cathode ray tube having a screen comprising A. a plurality of separate series-connected, coiled conductive ribbons spaced closely along an axis and arranged to engage around the neck of the tube,

B. means for supplying a dc. current to said ribbons, the dimensions of each said ribbon being selected so as to produce an ampere turns distribution along said axis so as to bring substantially all offaxis electrons to a single focal point on the tube screen,

C. an additional conductive ribbon coiled coaxially with said other ribbons, said additional ribbon being situated midway along said axis and said ribbons progressively further away from said additional ribbon having progressively increasing dimensions, and

D. means for applying a current signal to said additional ribbon which varies in accordance with the deflection of the beam.

16. A lens system as defined in claim 15 and further including a length of semiconductive tape coiled up with each said ribbon so as to constitute a shunt resistance therefor.

17. A lens system as defined in claim 15 and further including a ferrite sleeve encircling said coiled ribbons so as to confine the magnetic field within the tube.

18. A lens system as defined in claim 15 and further including control means connected to receive vertical and horizontal deflection signals, said control means including a signal shaping circuit which shapes the signal applied to said additional ribbon so that it has a generally parabolic waveform as the electron beam is deflected off the tube axis.

19. A lens system as defined in claim 15 and further including discs of semiconductive material mounted between and separating each of said ribbons and coupled to each of said ribbons so as to function as a shunt resistance therefor.

20. A magnetic focus lens system for focusing an electron beam generated in a cathode ray tube comprising A. a plurality of separate, toroidal, permanent magnet segments spaced closely along an axis arranged to engage around the neck of said tube, said magnets being arranged and distributed along said axis so as to bring substantially all off-axis electrons to a single focal point on said screen,

B. a toroidal winding wound coaxially with said mag- C ih eans for producing deplection signals for deflecting the electron beam, and

D. means responsive to said deflection signals for applying a current signal to said toroidal winding which varies in accordance with the deflection of said beam.

21. A lens system as defined in claim 20 and further including a ferrite sleeve encircling said magnets and winding so as to confine the magnetic field within the tube.

esmmmrs Patent No. 3,686, 527 wee" i f m 1972 Inventofls) William D. Gabor It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 Line 22 change "26b" to --36b-.

Column 5 line-l9 change "applied to -applies--.

Column 6 line 4-5 change "the" to --this--.

Column 6 line 54 after "coupled" insert --less--.

Eolumn 12 line 55 I change "2. to -1.

Signed and sealed this 29th day of May 1973.

(SEAL) Attest:

EDWARD MY.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents Po-woso TEFEQATE "0 F D855 Ailgiljst 22;. 197-2 IBVGI'ILCQIKS) WiHiam D. Gabor It is certified that error appears in the above-identified patentand thatsaid Letters Patent are hereby corrected as shown below:

Column 4 Line 22 Column 5 Erie-.19 Column 6 line 45 Column 6 line 54 Column 12 line 55 Signed and'sealed this 29th day of May 19 73.

(SEAL) Attest:

V ROBERT GOTTSCHALK Commissioner of Patents EDWARD M.FLETCH ER,J-R.- Attesting Officer 

1. A magnetic focus lens system for focusing an electron beam generated in a cathode ray tube comprising A. a plurality of separate windings spaced closely along an axis and arranged to engage around the neck of said tube, B. means for supplying a d.c. current to said windings, the ampere turns of said windings being distributed along said axis so as to bring substantially all off-axis electrons to a single focal point on said screen, C. an additional winding wound coaxially with said windings, D. means for producing deflection signals for deflecting the electron beam, and E. means responsive to said deflection signals for applying a current signal to said additional winding which varies in accordance with the deflection of said beam.
 2. A magnetic focus lens system as defined in claim 1 and further including a bobbin A. for engaging over the neck of said tube, and B. having separate compartments for containing said windings.
 2. beveled at an angle of approximately 45*- 135* measured from the radius of said sleeve, C. means for producing deflection signals for deflecting the electron beam, and D. means responsive to said deflection signals for varying the focal length of the lens so as to maintain the beam in focus as it moves across said surface.
 2. means for applying a current signal to said winding which varies approximately as the square root of the distance which the beam is deflected from the beam axis.
 3. A magnetic lens system as defined in claim 2 wherein A. said additional winding is situated in a compartment near the middle of said bobbin, and B. said windings progressively further away from said additional winding have progressively increasing numbers of winding turns.
 4. A lens system as defined in claim 1 and further including a ferrite sleeve encircling said windings so as to confine the magnetic field within the tube.
 5. A lens system as defined in claim 4 wherein the opposite ends of said sleeve are shaped to give the end field magnetic force lines generally parabolic curvature.
 6. A lens system as defined in claim 1 and further including A. a pair of small trim windings situated adjacent the outer ones of said windings, and B. an adjustable current source connected to said trim windings for shaping the magnetic force lines of the ends of said lens to give them generally parabolic curvature.
 7. A lens system as defined in claim 1 wherein said signal applying means comprises A. means for dynamically damping said windings, and B. control means connected to receive vertical and horizontal deflection signals, said control means including a signal shaping circuit which shapes the signal applied to said additional winding so that it has a generally parabolic waveform as the electron beam is deflected off the tube axis.
 8. A lens system as defined in claim 1 wherein said windings comprise a plurality of separate current-carrying coiled ribbons arranged along the axiS of the beam.
 9. A lens as defined in claim 8 and further including a sleeve of low magnetic permeability material encircling said ribbons for confining the focus field within the lens.
 10. A lens as defined in claim 8 and further including a length of semi-conductive tape rolled up with each said ribbon so as to function as a shunt resistance therefor.
 11. A lens as defined in claim 8 and further including discs of semi-conductive material mounted between and separating each of said ribbons and coupled to each of said ribbons so as to function as a shunt resistance therefor.
 12. A focus system for focusing an electron beam comprising A. a magnetic focus lens for producing a static magnetic field along the axis of the beam to focus the beam on a surface, B. means for producing deflection signals for deflecting the electron beam, and C. means responsive to said deflection signals for varying the focal length of the lens so as to maintain the beam in focus as it moves across said surface, said focal length varying means comprising
 13. A focus lens for focusing an electron beam on a surface comprising A. a plurality of magnetic field-producing elements spaced along the axis of the beam so as to generate a magnetic force field whose density varies along said axis and radially outwardly therefrom in a manner to focus off-axis electrons in the beam at a single focal point on said surface, B. a sleeve of low magnetic permeability material encircling said field-producing elements for confining the focus fields within the lens, said sleeve having its opposite ends
 14. A lens as defined in claim 13 wherein said sleeve has a barrel-shaped inside contour.
 15. A magnetic focus lens system for focusing an electron beam generated in a cathode ray tube having a screen comprising A. a plurality of separate series-connected, coiled conductive ribbons spaced closely along an axis and arranged to engage around the neck of the tube, B. means for supplying a d.c. current to said ribbons, the dimensions of each said ribbon being selected so as to produce an ampere turns distribution along said axis so as to bring substantially all off-axis electrons to a single focal point on the tube screen, C. an additional conductive ribbon coiled coaxially with said other ribbons, said additional ribbon being situated midway along said axis and said ribbons progressively further away from said additional ribbon having progressively increasing dimensions, and D. means for applying a current signal to said additional ribbon which varies in accordance with the deflection of the beam.
 16. A lens system as defined in claim 15 and further including a length of semiconductive tape coiled up with each said ribbon so as to constitute a shunt resistance therefor.
 17. A lens system as defined in claim 15 and further including a ferrite sleeve encircling said coiled ribbons so as to confine the magnetic field within the tube.
 18. A lens system as defined in claim 15 and further including control means connected to receive vertical and horizontal deflection signals, said control means including a signal shaping circuit which shapes the signal applied to said additional ribbon so that it has a generally parabolic waveform as the electron beam is deflected off the tube axis.
 19. A leNs system as defined in claim 15 and further including discs of semiconductive material mounted between and separating each of said ribbons and coupled to each of said ribbons so as to function as a shunt resistance therefor.
 20. A magnetic focus lens system for focusing an electron beam generated in a cathode ray tube comprising A. a plurality of separate, toroidal, permanent magnet segments spaced closely along an axis arranged to engage around the neck of said tube, said magnets being arranged and distributed along said axis so as to bring substantially all off-axis electrons to a single focal point on said screen, B. a toroidal winding wound coaxially with said magnets. C. means for producing deflection signals for deflecting the electron beam, and D. means responsive to said deflection signals for applying a current signal to said toroidal winding which varies in accordance with the deflection of said beam.
 21. A lens system as defined in claim 20 and further including a ferrite sleeve encircling said magnets and winding so as to confine the magnetic field within the tube. 